Field of the Invention
The invention concerns a method for acquiring a magnetic resonance data set from a target area of a patient containing least one metal object that distorts the basic magnetic field of the magnetic resonance scanner due to susceptibility differences, wherein a slice selection gradient rising in one direction is used to select a slice to be acquired. The invention also relates to a magnetic resonance apparatus and a storage medium for implementing such a method.
Description of the Prior Art and Related Subject Matter
Magnetic resonance imaging is a well-known imaging modality. Radio-frequency excitations are used in order to deflect nuclear spins in an examination subject situated in a basic magnetic field (B0 field) in order to measure signals resulting therefrom. The radio-frequency field of the radio-frequency excitation is generally designated a B1 field. Gradient fields are also used in such imaging. These are superimposed on the basic magnetic field, generally as a linear gradient field. A differentiation is made between the slice selection gradient, the phase-encoding gradient and the read-out gradient. If the magnetic resonance recording is to be performed slice-by-slice, such as is the case with the acquisition method discussed herein, it is, possible to superimpose a slice-selection gradient field so that the overall magnetic field in the homogeneity region rises in one direction. The result is a change in the resonance frequency along the slice-selection direction. Therefore, if a radio-frequency excitation with a specific bandwidth is irradiated, in an ideal case only a clearly determined slice is excited in which the resonance frequencies lie within the excited frequency band.
Problems occur in magnetic resonance examinations when metallic objects are present in the target area, for example metallic implants in a patient. Despite the associated complications, examination of patients with metallic implants has become an important application. In particular, the increasing number of patients with orthopedic implants, for example screws, fixations, artificial joints etc., has led to the development of new techniques designed to reduce the significant image distortions due to such metals, since the high soft-tissue contrast achieved with magnetic resonance imaging is superior to other examination methods. Here, it should be taken into account that other imaging modalities, for example computed tomography, also exhibit severe metal artifacts.
In magnetic resonance imaging, when a metallic object is present in the target area, the image artifacts are primarily caused by the distortion of the static magnetic basic field (B0 field), which in turn is attributable to the great difference in the magnetic susceptibility between body tissue and metal. When acquiring magnetic resonance data from slices, in particular slices in a slice stack, covering the target area, the inhomogeneities of the basic magnetic field also result in a spatial distortion of the slices in the slice-selection direction so that reliable diagnosis by the doctor is frequently not possible with uncorrected magnetic resonance images. The distortion or geometric displacement of the slice that occurs due to the inhomogeneities of the basic magnetic field is proportional to the resonance frequency deviation. The resonance frequency deviation (often also designated the off-resonance frequency) corresponds to the frequency difference between the actual or theoretical resonance frequency of the spins at a specific location, and the actual resonance frequency of these spins. Here, the theoretical resonance frequency of the spins is that which would occur with an optimally homogeneous basic magnetic field, the actual resonance frequency is obtained due to the distortion of the basic magnetic field.
In the prior art, methods have been suggested in order, despite the distortion of the basic magnetic field and hence the slices, to resolve and correctly assign all the information belonging to a slice (distortion correction). One known promising method for this is additionally to switch a phase-encoding gradient in the slice-selection direction. One method based on a two-dimensional TSE sequence is the so-called “SEMAC” method, which, for example, is described in US 2010/0033179 A1 or the underlying article by Wenmiao Lu et al. “SEMAC: Slice Encoding for Metal Artifact Correction in MRI”, Magnetic Resonance in Medicine 62: 66-76 (2009). For successful SEMAC correction, it is necessary to select both the number of slices and the additional phase-encoding steps such that the spatial and spectral extensions of the field distortions are completely covered. This entails a significant prolongation of the measuring (data acquisition) time.
Therefore, methods have been suggested for determining the number of phase-encoding steps required in the slice-selection direction by means of a scout scan in order to restrict them to the minimum necessary size. Examples of this can be found in the article by B. A. Hargreaves et al. “Adaptive Slice Encoding for Metal Artifact Correction”, Proc. Intl. Soc. Mag. Reson. Med. 18 (2010), page 3083, and in particular the subsequently published German patent application DE 10 2013 205 930.2 and U.S. application 61/918,786. For example, DE 10 2013 205 930.2 describes a scout scan that can be performed extremely quickly with which the read-out gradient is applied in the slice-selection direction, i.e. in the same direction as the slice selection gradient, wherein the slice selection gradient and the read-out gradient have different polarities such that an overall shift of the slice in the same direction is obtained which can be measured.
However, even when using correction methods of this kind, the edge slices of the slice stack present problems. The radiologist will generally place the slice stack as closely as possible around the region of interest because a higher number of layers increases the SAR loading on the patient and hence possibly the measuring time, which is anyway already much longer when using multiple phase-encoding steps in the slice-selection direction. Thus, particularly with strong distortion in edge slices, it can occur that off-resonant spins are not acquired by the excitation of another slice and therefore also cannot be reconstructed when using methods such as SEMAC. Depending upon the distortion of the basic magnetic field and the location of the metal object, spins can be excited which, although their resonance frequency deviations are the same, have different +/− signs. Then, it can happen that, in the case of symmetrical structures, for example, the head of a hip implant, more details are depicted in one direction than in the other direction. So far, no solution to this problem has been known apart from the significant prolongation of the measuring time if a number of (actually unnecessary) additional slices is recorded.